How to check how much GPU memory is used by object?
Objects like trees or toothbrushes take up much memory.
For trees, even games use object instancing. While copying the tree, use Alt + D. This will keep you from editing the trees individually but will keep the memory requirement low, you can also have a few trees and create an illusion by rotating them, (fake it!)

Too many leafs will unevitably result in high memory need. I strongly recommend you to take a look to this cycles instancing test.

If you don't want to lower the details, you can try caching the required files on your computer(cycles only). To do this, you need to use the (Cache BVH) checkbox under the "performance" section of render tab.
User preferences also let you to adjuct the available RAM to use.